The chronic myeloid leukemia stem cell: stemming the tide of persistence

Chronic myeloid leukaemia (CML) is caused by the acquisition of the tyrosine kinase BCR-ABL1 in a haemopoietic stem cell (HSC), transforming it into a leukaemic stem cell (LSC) that self-renews, proliferates and differentiates to give rise to a myeloproliferative disease. While tyrosine kinase inhibitors (TKI) that target the kinase activity of BCR-ABL1 have transformed CML from a once fatal disease to a manageable one for the vast majority of patients, only ~10% of those who present in chronic phase (CP) can discontinue TKI treatment and maintain a therapy-free remission. Strong evidence now shows that CML LSC are resistant to the effects of TKIs and they persist in all patients on long-term therapy, where they may promote acquired TKI resistance, drive relapse or disease progression and inevitably represent a bottleneck to cure. Since their discovery in patients almost two decades ago, CML LSC have become a well-recognised exemplar of the cancer stem cell and have been characterised extensively with the aim of developing new curative therapeutic approaches based on LSC eradication. This review summarises our current understanding of many of the pathways and mechanisms that promote the survival of the CP CML LSC and how they can be a source of new gene coding mutations that impact in the clinic. We also review recent pre-clinical approaches that show promise to eradicate the LSC, and future challenges on the path to cure

Celecoxib inhibits proliferation and survival of chronic myelogeous leukemia (CML) cells via AMPK-dependent regulation of β-catenin and mTORC1/2.

CML is effectively treated with tyrosine kinase inhibitors (TKIs). However, the efficacy of these drugs is confined to the chronic phase of the disease and development of resistance to TKIs remains a pressing issue. The anti-inflammatory COX2 inhibitor celecoxib has been utilized as anti-tumour drug due to its anti-proliferative activity. However, its effects in hematological malignancies, in particular CML, have not been investigated yet. Thus, we tested biological effects and mechanisms of action of celecoxib in Philadelphia-positive (Ph+) CML and ALL cells.We show here that celecoxib suppresses the growth of Ph+ cell lines by increasing G1-phase and apoptotic cells and reducing S- and G2-phase cells. These effects were independent of COX2 inhibition but required the rapid activation of AMP-activated protein kinase (AMPK) and the consequent inhibition mTORC1 and 2. Treatment with celecoxib also restored GSK3β function and led to down-regulation of β-catenin activity through transcriptional and post-translational mechanisms, two effects likely to contribute to Ph+ cell growth suppression by celecoxib.Celecoxib inhibited colony formation of TKI-resistant Ph+ cell lines including those with the T315I BCR-ABL mutation and acted synergistically with imatinib in suppressing colony formation of TKI-sensitive Ph+ cell lines. Finally, it suppressed colony formation of CD34+ cells from CML patients, while sparing most CD34+ progenitors from healthy donors, and induced apoptosis of primary Ph+ ALL cells.Together, these findings indicate that celecoxib may serve as a COX2-independent lead compound to simultaneously target the mTOR and β-catenin pathways, key players in the resistance of CML stem cells to TKIs

Targeting self-renewal pathways in myeloid malignancies

A fundamental property of hematopoietic stem cells (HSCs) is the ability to self-renew. This is a complex process involving multiple signal transduction cascades which control the fine balance between self-renewal and differentiation through transcriptional networks. Key activators/regulators of self-renewal include chemokines, cytokines and morphogens which are expressed in the bone marrow niche, either in a paracrine or autocrine fashion, and modulate stem cell behaviour. Increasing evidence suggests that the downstream signaling pathways induced by these ligands converge at multiple levels providing a degree of redundancy in steady state hematopoiesis. Here we will focus on how these pathways cross-talk to regulate HSC self-renewal highlighting potential therapeutic windows which could be targeted to prevent leukemic stem cell self-renewal in myeloid malignancies

Chronic myeloid leukemia as a stem cell-derived malignancy

Chronic myeloid leukemia (CML) is a myeloproliferative disease of the hematopoietic stem cells, characterized by the presence of the Philadelphia (Ph) chromosome. Although imatinib inhibits the BCR-ABL kinase activity, clinical experiences confirm that imatinib may not target CML stem cells in vivo. The identification of signaling pathways responsible for the self-renewal properties of leukemic stem cells in CML will help in the discovery of novel therapeutic targets. Here we review signaling pathways including Wnt/β-catenin, Hedgehog, Alox5, and Foxo which play crucial roles in the maintenance of stem cell functions in CML. It is thought that inhibition of key genes that are part of self-renewal associated signaling pathways may provide an effective way to reduce aberrant stem cell renewal in CML

Critical Molecular Pathways in Cancer Stem Cells of Chronic Myeloid Leukemia: A Dissertation

Chronic myeloid leukemia (CML) is a disease characterized by the expansion of granulocytic cells. The BCR-ABL tyrosine kinase inhibitor imatinib, the frontline treatment for Ph+ leukemias, can induce complete hematologic and cytogenetic response in most chronic phase CML patients. Despite the remarkable initial clinic effects, it is now recognized that imatinib will unlikely cure patients because a small cell population containing leukemic stem cells (LSCs) with self-renewal capacity is insensitive to tyrosine kinase inhibitors. In Chapter I, I briefly review the BCR-ABL kinase and its related signaling pathways. BCR-ABL kinase activates several signaling pathways including MAPK, STAT, and JNK/SAPK. BCR-ABL also mediates kinase-independent pathways through SRC family kinases. I will also discuss pathways involving β-catenin, hedgehog, FoxO and Alox5 are critical to the regulation of self-renewal and differentiation in LSC of CML. As detailed in Chapter II, I describe our work evaluating the effects of omacetaxine, a novel CML drug inducing cell apoptosis by inhibition of protein synthesis, on self-renewal and differentiation of LSCs and BCR-ABL-induced CML and acute lymphoblastic leukemia (B-ALL) in mice. We found that treatment with omacetaxine decreased the number of LSCs and prolonged the survival of mice with CML or B-ALL. In chapter III, I describe that Alox5 is an essential gene in the function of LSCs and CML development. We show evidence that Alox5 affects differentiation, cell division, and survival of long-term LSCs. Treatment of CML mice with a 5-LO inhibitor also impaired the function of LSCs similarly and prolonged survival. In chapter IV, I present evidence of our work showing a further dissection the Alox5 pathway by comparing the gene expression profiles of wild type and Alox5-/- LSCs. We show that Msr1 deletion causes acceleration of CML development. We also show that Msr1 affects CML development by regulating the PI3K-AKT pathway and β-catenin. Taken together, these results demonstrate that some pathways including Alox5 and Msr1 play an important role in regulating the self-renewal and differentiation of LSC. More efforts should be put into developing the novel strategies that may effectively target LSCs and thus cure CML

Arachidonate 15-lipoxygenase is required for chronic myeloid leukemia stem cell survival

Cancer stem cells (CSCs) are responsible for the initiation and maintenance of some types of cancer, suggesting that inhibition of these cells may limit disease progression and relapse. Unfortunately, few CSC-specific genes have been identified. Here, we determined that the gene encoding arachidonate 15-lipoxygenase (Alox15/15-LO) is essential for the survival of leukemia stem cells (LSCs) in a murine model of BCR-ABL-induced chronic myeloid leukemia (CML). In the absence of Alox15, BCR-ABL was unable to induce CML in mice. Furthermore, Alox15 deletion impaired LSC function by affecting cell division and apoptosis, leading to an eventual depletion of LSCs. Moreover, chemical inhibition of 15-LO function impaired LSC function and attenuated CML in mice. The defective CML phenotype in Alox15-deficient animals was rescued by depleting the gene encoding P-selectin, which is upregulated in Alox15-deficient animals. Both deletion and overexpression of P-selectin affected the survival of LSCs. In human CML cell lines and CD34+ cells, knockdown of Alox15 or inhibition of 15-LO dramatically reduced survival. Loss of Alox15 altered expression of PTEN, PI3K/AKT, and the transcription factor ICSBP, which are known mediators of cancer pathogenesis. These results suggest that ALOX15 has potential as a therapeutic target for eradicating LSCs in CML

The Molecular Mechanisms for Maintenance of Cancer Stem Cells in Chronic Myeloid Leukemia: A Dissertation

Chronic myeloid leukemia (CML) is a clonal hematopoietic stem cell disorder associated with the Philadelphia chromosome (Ph) that arises from a reciprocal translocation between chromosomes 9 and 22, thereby resulting in the formation of the chimeric BCR-ABL oncogene encoding a constitutively activated tyrosine kinase. BCR-ABL tyrosine kinase inhibitors (TKIs) induce a complete hematologic and cytogenetic response in the majority of chronic phrase CML patients. However, TKIs cannot efficiently eradicate leukemia stem cells (LSCs) because of the insensitivity of LSCs to TKIs. Therefore, developing new strategies to target LSCs is necessary and critical for curing CML, and success of this approach depends on further understanding the molecular mechanisms by which LSCs survive and are maintained. In Chapter I, I briefly introduce CML disease, BCR-ABL oncoprotein, and TKIs. I also describe the identification and features of LSCs. Several key pathways in LSCs including Wnt/ß-catenin, hedgehog, FoxO, Bcl6 and HIF1, are discussed. I also propose our strategy to identify unique molecular pathways that are important for LSCs but not their normal stem cell counterparts. In Chapter II, I describe our finding about the function of the positive regulator, HIF1α, in CML development and LSC survival. I show that loss of HIF1α impairs the maintenance of CML through impairing cell cycle progression and inducing apoptosis of LSCs, and I also report that p16Ink4a and p19Arf mediate the effect of HIF1α on LSCs, as knockdown of p16Ink4a and p19Arf rescues the defective colony-forming ability of HIF1α-/- LSCs. As detailed in Chapter III and IV, through comparing the global gene expression profiles of LSCs and HSCs, I find two novel regulators, Blk and Scd1, which act as tumor suppressors in CML development. In Chapter III, I show that Blk is markedly down-regulated by BCR-ABL in LSCs, and that c-Myc and Pax5 mediate this down-regulation. Deletion of Blk accelerates CML development; conversely, Blk overexpression significantly delays the development of CML and impairs the function of LSCs. I also demonstrate that p27, as a downstream effector, is involved in the function of Blk in LSCs. Blk also functions as a tumor suppressor in human CML stem cells, and inhibits the colony-forming ability of human CML cells. In Chapter IV, I investigate the function of another negative regulator, Scd1, in CML LSCs, and find that expression of Scd1 is down-regulated in mouse LSCs and human CML cells. We report that Scd1 acts as a tumor suppressor in CML, as loss of Scd1 causes acceleration of CML development and overexpression of Scd1 delays CML development. Using a colony-forming assay, I demonstrate that Scd1 impairs the maintenance of LSCs due to the change of expression of Pten, p53 and Bcl2. Importantly, I find that both Blk and Scd1 do not affect normal hematopoietic stem cells (HSCs) or hematopoiesis. Taken together, our findings demonstrate that HIF1α is required for the maintenance of CML LSCs, and conversely that Blk and Scd1 suppress the function of LSCs, suggesting that combining TKI treatment with specific targeting of LSCs will be necessary for curing CML

Understanding the role of bone marrow niche in myeloid malignancies

Normal hematopoiesis is generated and maintained by rare hematopoietic stem cells (HSCs) through their capacity to self-renew and differentiate. This process is rigorously controlled, both by HSC-intrinsic molecular programs and extrinsic signals emitted by the local bone marrow (BM) microenvironment, the so-called HSC niche. The BM niche consists of many cellular elements, including mesenchymal stem cells (MSCs), and soluble factors secreted by the cells. The niche homeostasis is critical for maintenance of normal hematopoiesis, and disruption of this BM niche may lead to malignant hematopoiesis, including leukemia. On the other hand, once malignant hematopoiesis is established, the niche structure and composition can be altered to protect leukemia-initiating stem cell (LSC). The aims of the presented thesis were to investigate the role of the BM niche in development of myeloid malignancies. In study I, we analyzed expression of leukotriene (LT) signaling molecules in LSCs derived from chronic myeloid leukemia (CML) patients, and tested their response to pharmacological inhibition of LT signaling. By using single cell PCR, we found only low expression of ALOX5 in patient BCR-ABL+ LSCs and BCR-ABL- HSCs. Moreover, in contrast to previous observations in mice and in liquid cultures in vitro, pharmacological inhibition of ALOX5 did not result in any significant growth suppression of CML LSCs in long-term culture initiating cell (LTC-IC) assay on a stromal cell layer. Furthermore, although expression of CYSLT1 was detected in the majority of analyzed LSCs, treatment with its antagonist, montelukast, did not significantly reduce the LTC-IC activity of LSCs. Thus, these results suggest that pharmacological inhibition of the LT pathway might not be sufficient to eradicate LSCs, particularly in the presence of BM stromal cells. In study II, we investigated the role of BM niche in pathogenesis of MDS/MPN by using a Sipa1-/- mouse model. We found that Sipa1 was expressed in BM stromal cells from mice and healthy humans, but was downregulated in these cells from patients with MPN and MDS/MPN. Additionally, Sipa1 deficiency in mice led to phenotypical and functional alterations in the BM cellular niche prior to disease development, and reciprocal transplantation experiments further confirmed that Sipa1-/- BM niche was a prerequisite for MDS/MPN development. Moreover, RNA sequencing analysis showed dysregulated expression of inflammatory cytokines and growth factors in the BM stromal cells from young, disease-free Sipa1-/- mice. Altogether, our data suggest that Sipa1 expression in the BM stromal cells is critical for maintaining BM niche homeostasis, and that Sipa1 deficiency in BM niche plays an instructive role in development of MDS/MPN in mice. Finally in study III, we prospectively characterized BM stromal cells in newly-diagnosed patients with CML. First of all, we discovered that patient’s BM stromal cells share similar immunophenotype as normal BM (NBM) counterparts, but that the CML BM niche composition was changed, showing increased frequency of endothelial cells. Moreover, we found alterations in functional properties of CML-derived MSCs, e.g. an impaired osteochondrogenic differentiation potential, and an enhanced capacity to support NBM hematopoietic stem and progenitor cells in vitro. Even though no BCR-ABL fusion gene was detected in CML BM stromal cells, the RNA sequencing revealed cytokine dysregulation, particularly loss of CXCL14 in CML BM niche. Interestingly, restoration of CXCL14 expression in stromal cells suppressed the growth of LSCs in LTC-IC assays, but promoted their differentiation. These results indicate that CXCL14 might help to eradicate LSCs and therefore serve as a new therapeutic candidate for CML treatment. To conclude, we herein showed that BM niche might contribute to myeloid malignancies in mice and human. Thus, targeting the dysregulated BM niche factors and the abnormal interaction between BM niche and LSCs could be a promising therapeutic approach to treat patients with myeloid malignancies